Interesting you ask. We live here at about 6200 feet- not even considered high altitude. But, we get A LOT of women in their 3rd trimester who come up here from the Bay area (sea level) and go into preterm labor.
Not sure if it's from altitude/lower oxygen or dehydration which happens a lot faster here or combo, but despite studies that don't show this, we see it. We haven't kept track of numbers, so this is purely speculation and anecdotal, but it's not uncommon for us. I talk about this with the OBs I work with and they say it sure seems like more preterm labor in women traveling to altitude than those who live here.
Because studies have apparently had conflicting results with some showing no risk of problems traveling to altitude (disclaimer: I have not reviewed the studies, so this is hearsay from others), a lot of doctors tell their patients it's ok to travel to Tahoe. But, they get here, go into preterm labor, which can usually be stopped with hydration and rest, and get sent right back where they came from!